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The Richmond and Greenwich Slices of the Hamburg Klippe In Eastern Pennsylvania Stratigraphy, Sedimentology, Structure, and Plate Tectonic Implications By GARY G. LASH and AVERY ALA DRAKE, JR.
GEOLOGICAL
SURVEY
PROFESSIONAL
PAPER
1312
A stratigraphies sedimentologic, and structural stud}/ of a major Taconic-type allochthon in the Great Valley of eastern Pennsylvania
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984
DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director
Library of Congress Cataloging in Publication Data Lash, Gary George. The Richmond and Greenwich slices of the Hamburg klippe in Eastern Pennsylvania. (Geological Survey professional paper ; 1312) Bibliography: p. Supt. of Docs, no.: 119.16:1312 1. Nappes (Geology) Pennsylvania Hamburg region. I. Drake, Avery Ala, 1927III. Series. QE606.5.U6L375 1983 551.8'7 83-600340
. II Title.
For sale by the Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 Any use of trade names and trademarks in the publication is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
CONTENTS Page
Page
Abstract................................................................................................ Introduction ........................................................................................ Previous work .................................................................................... Geologic framework .......................................................................... Terminology ....................................................................................... Stratigraphy and sedimentology ............................................... Windsor Township Formation .................................................. Weisenberg Member .............................................................. Switzer Creek Member ............................................................ Dreibelbis Member ................................................................. Miscellaneous units .................................................................. Graywacke petrography ...................................................... Provenance ................................................................................. Age and correlation .................................................................. Environment of deposition ....................................................
1 2 2 2 3 3 3 3 6 7 9 11 12 13 15
Stratigraphy and sedimentology Continued Virginville Formation ...................................... .................................................... .................................................... Sacony Member .............................................. Onyx Cave Member ...................................... .................................................... .................................................... Moselem Member .......................................... Age and correlation .............................................................. Environment of deposition ................................................. Paleogeography ..................................................................... Structure ................................ Rock fabric ........................ Faults .................................. Style of deformation ........ Plate tectonic implications Conclusions ........................... References cited .................
17 17 18 20 21 22 24 26 26 27 27 29 35 37
ILLUSTRATIONS Page
FKJURE 1. 2. 3. 4-14.
15. 16. 17. 18. 19-25.
26. 27. 28. 29. 30.
Geologic maps of eastern Pennsylvania .......................... ...................................................................................................................... Stratigraphic diagram of the Lehigh Valley sequence .............. .................................................................................................. Bouma sequence and variations ................................ ........................................................................................................................... Photographs showing: 4. Contorted beds and faults in siltstone of the Weisenberg Member ................................................................................. 5. Sequences of graywacke, siltstone, and mudstone of the Dreibelbis Member ............................................................ 6. Groove casts on the sole of a thick graywacke bed ........................ .......... ....................................................................... 7. Graywackes with thin pelitic intercalations .......................................................................................................................... 8. Flute casts on the sole of a thin Tc_t, turbidite ........................................................................................................................ 9. Contorted graywacke beds .......................................................................................................................................................... 10. Thick-bedded chert interbedded with red and light green shale and mudstone ......................................................... 11. Limestone megaclast ..................................................................................................................................................................... 12. Chaotic slump folds in interbedded mudstone and chert .............................................................................................. 13. Graywacke megaclast in boulder conglomerate ................................................................................................................. 14. Irregularly shaped graywacke megaclast in boulder conglomerate ............................................................................... Classification diagram for the graywackes of the Windsor Township Formation .................................................................... Triangular plot comparing detrital modes ........................................................................................................................................... Correlation diagram for the Lehigh Valley sequence, Jutland klippe, and Windsor Township Formation ........................ Depositional model for the Windsor Township Formation ................................................................................................................ Photographs showing. 19. Ovoid masses of Sacony Member siltstone .............................................................................................................................. 20. Peloidal limestone filling channels in black shale and lime mud ...................................................................................... 21. Sedimentary boudinage in ribbon limestones ........................................................................................................................ 22. Polymict carbonate-clast conglomerate .................................................................................................................................. 23. Polymict carbonate-clast conglomerate containing clasts oriented perpendicular to bedding ................................ 24. Contorted beds in interlaminated shale and dolostone ................................... .................................................................... 25. Moselem Member mudstone and shale ................................................................................................................................... Correlation diagram for the Lehigh Valley sequence, the Windsor Township Formation, the Virginville Formation, and the Jutland klippe .............................................................................................................................................. Facies diagram for the Sacony, Onyx Cave, and Moselem Members of the Virginville Formation ........................................ Photograph showing soft-sediment folds in ribbon limestones of the Moselem Member ............................................................ Plot of soft-sediment fold axes, fold vergence, slip plane, and slip-line orientation of slump folds in the Moselem Member .................................................................................................................................................................... Plot of fold axes, vergence, and slip-line orientation of tectonic folds in the Moselem Member ..............................................
ni
4 6 6 7 7 8 8 8 9 9 10 10 10 11 12 14 15 16 18 19 19 19 20 21 21 22 24 25 25 26
IV
CONTENTS Page
31-33. Photographs showing: 31. Large boudin or phacoid in a pelitic matrix ...................... ........................ ........................................ ....................... 32. Small graywacke boudin in a pelitic matrix ....... .............................................................................................................. 33. Pinch-and-swell structure ........... .............................. . .............................................................. .. ............................ 34. Plate tectonic model for the Hamburg klippe ................................................ ................................. ...... ...................................
28 28 29 30
TABLE Page
TABLE 1. Modal analyses of graywackes from the Windsor Township Formation ........................................................................................
13
THE RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIASTRATIGRAPHY, SEDIMENTOLOGY, STRUCTURE, AND PLATE TECTONIC IMPLICATIONS By GARY G. LASH 1 and AVERY ALA DRAKE, JR.
ABSTRACT Taconic-type klippen, such as those in the northern and maritime Appalachians, are also characteristic of the Taconides of the central Appalachians. One such body of rock, the Hamburg klippe, anomalous with respect to the age and lithology of its surroundings, has long been recognized in the Great Valley of east-central Pennsylvania. The Hamburg klippe is bounded on the southeast by the parautochthonous rocks of the Lehigh Valley sequence of Cambrian and Ordovician age. The carbonate rocks of this sequence were deposited on the southeast-facing shelf of the North American Continent and, with the underlying Proterozoic Y basement, were deformed into the Musconetcong nappe system during the Taconic orogeny. Rocks of the parautochthonous nappe system were thrust onto the klippe during the Alleghanian orogeny. To the north, the klippe is in fault contact with both the Shochary Ridge sequence of Ordovician age and the Shawangunk Formation of Late Ordovician(?) or younger age. Recent detailed mapping at the eastern end of this klippe, between the Schuylkill River and Kutztown in eastern Pennsylvania, has led to the recognition of two stratigraphically and structurally different slices. The lower Greenwich slice consists of rocks of the Windsor Township Formation. The Windsor Township is a Middle Ordovician flysch sequence that contains various-sized bodies of Lower Ordovician chert, deepwater limestone, and variegated shale. The Windsor Township (about 6,215 m thick) consists of three members: (1) the Weisenberg (1,740 m thick), a gray-green shale unit that contains local conglomerate beds; (2) the Switzer Creek (815 m thick), a gray wacke-gray-green shale unit that contains local pebble conglomerate; and (3) the Dreibelbis (3,660 m thick), a graywacke-gray-green shale unit. The sedimentology and stratigraphy of these rocks suggest that the graywacke units (Dreibelbis and Switzer Creek) were deposited as channel-axis grainflow and turbidite sediments and as interchannel turbidite sediments, whereas rocks of the Weisenberg (the shale unit) were deposited as levee and open-fan muds adjacent to the channels. The rocks of the Windsor Township Formation were deposited in the middle-fan area of a large submarine fan, which had a continental source to the southeast. The older lenticular bodies of chert, deepwater limestone, and variegated shale are not restricted to any particular member but are found throughout the Windsor Township. These rocks probably were deposited in an abyssal environment that received periodic influxes of terrigenous material. They were emplaced later into the younger fan deposits. The upper Richmond slice contains rocks of the Virginville Formation of Late Cambrian and Early Ordovician age. This unit is divided into three members: (1) the Sacony (245m thick), consisting
1 Department of Geology. State University of New York College at Fredonia. Fredonia, NY 14063.
of olive-green siltstone and shale; (2) the Onyx Cave (90 m thick), consisting of massive to laminated calcarenite, ribbon limestone, massive carbonate-clast conglomerate, and interlaminated black shale and orange dolostone; and (3) the Moselem (230 m thick), consisting of well-cleaved black and green argillite and mudstone and lesser proportions of ribbon limestone and carbonate-clast conglomerate. The Onyx Cave conformably overlies the Sacony, and both tectonically overlie the Moselem. The sedimentology and stratigraphy of these rocks suggest that (1) they were deposited near the base of a northwest-facing slope and (2) the Moselem is a distal equivalent of the Onyx Cave and was deposited to the northwest of it. We suggest that the rocks of the Virginville were deposited on a northwest-dipping slope adjacent to a shallow-water carbonate shelf. Subsequent to deposition, northwest-directed thrust faulting brought the Sacony and Onyx Cave over the Moselem. The rocks of the Hamburg klippe record the effects of three phases of deformation. Folds related to the first and third phases trend east-northeast to east-southeast and, generally speaking, are differentiated only where overprinting is recognized. The second deformation phase produced folds that trend northeast. Faults within the Greenwich and Richmond slices range from steeply to moderately dipping upthrusts to subhorizontal thrust faults and typically are not well exposed. The contact between the Richmond and Greenwich slices is a thrust fault marked by slivers of Greenwich slice rocks tectonically mixed into the lower part of the Richmond slice. Rocks in the Richmond slice and the parautochthon deformed differently than those in the Greenwich slice. The less competent rocks of the Richmond slice and surrounding parautochthon contain a slaty cleavage that is axially planar to folds formed during the first deformation (Di). The rocks of the Greenwich slice generally lack this cleavage but exhibit a scaly cleavage or a poorly developed fracture cleayage consisting of a group of intersecting fracture planes generally subparallel to bedding that formed prior to Di. The fabric of these rocks is similar to that of tectonic melanges formed by mechanical fragmentation and mixing. Results obtained in the present study, combined with data from other investigations within the Pennsylvania Great Valley, are consistent with a plate tectonic model that involves late Proterozoic rifting leading to the development of a marginal basin separating the North American craton from a microcontinent to the southeast. In Cambrian to early Middle Ordovician time, carbonate shelves developed on both the craton and microcontinent. The Virginville Formation was deposited on the lower slope adjacent to the shelf of the microcontinent. Southeast-dipping subduction of the North American craton, as well as any oceanic crust that may have lain to the southeast of it, beneath the microcontinent began in late Early Ordovician to early Middle Ordovician (Chazyan) time. Subduction was accompanied by (1) trench sedimentation of the flyschoid rocks of the Windsor Township Formation, (2) off-scraping and gravity
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA emplacement of the approaching older abyssal sediments, and (3) subduction-related deformation of all rocks of the Greenwich slice. The marginal basin was essentially closed by late Blackriveran to early Rocklandian time. Because the thick, buoyant continental crust of North America was unable to subduct beneath the microcontinent, the eastern margin of the North American craton was uplifted, exposing the miogeoclinal cover (Black River hiatus). At this time, deposition of the Jacksonburg Limestone and Martinsburg Formation exogeoclinal sequences began in response to uplift to the southeast. Stress continued to build up along the contact of the two plates during Rocklandian to Kirkfieldian time. Consequently, slices of continental basement from the eastern margin of North America were detached along southeasterly dipping ductile shear zones and were thrust into the overlying miogeoclinal sediments, resulting in the formation of a nappe system. The Richmond slice probably was thrust onto the Greenwich slice, and both were superposed on the forming nappe pile at this time of extreme crustal convergence. Continued northwest-directed movement of the nappes and Hamburg klippe during Shermanian time was followed by detachment of the allochthon from the advancing nappe pile and, finally, by its emplacement into the developing Martinsburg foredeep to the northwest.
INTRODUCTION
A large klippe consisting of several stacked slices is a major tectonic feature of the Taconide zone in the northern Appalachians (Zen, 1972). Taconic-type klippen are characteristic also of the Great Valley of the central Appalachians in New Jersey and especially eastern Pennsylvania (Drake, 1980). The eastern end of the largest klippe in Pennsylvania, the Hamburg klippe (Stose, 1946), was studied in the Kutztown and Hamburg 7%-minute quadrangles and surrounding areas (fig. 1). These quadrangles lie along the northern side of the Great Valley of eastern Pennsylvania. There, the Great Valley is bounded on the north by quartzites of Late Ordovician(?) to Silurian age that hold up Blue Mountain of the Appalachian Valley and Ridge. On the south, the Great Valley is bounded by the Proterozoic Y rocks of the Reading Prong. This paper describes, in detail, the stratigraphy, sedimentology, and structure of the rocks of the eastern end of the Hamburg klippe and speculates on the depositional environments and the plate tectonic implications of these rocks. PREVIOUS WORK
Allochthonous rocks in the Great Valley of Pennsylvania were recognized by Stose (1946, 1950a, b), Stose and Jonas (1927), and Kay (1941). These noted similarities between these rocks and rocks of the Taconic sequence of New York and New England.
Stose (1946) referred to these rocks as the "Hamburg klippe." Proponents of the Hamburg klippe maintained that the rocks are lithologically different and older than the surrounding Martinsburg Formation. Despite the arguments of Stose, Kay, and other geologists, Gray and Willard (1955) asserted that the variegated shale and associated rocks are facies equivalents of the parauthochthonous Martinsburg Formation and are not allochthonous. In addition, McBride (1962) in his study of the Martinsburg in New Jersey, Pennsylvania, and Virginia, recognized no allochthonous rocks. In the late 1960's and early 1970's, several geologists studied the Hamburg klippe. Field studies by Myers (1969) and Platt and others (1972) showed that there are, indeed, allochthonous elements within the Great Valley. These studies, supported by faunal evidence, have shown that the rocks of the Hamburg klippe are older than the surrounding Martinsburg Formation. An Early Ordovician age for the klippe rocks was indicated. Root (1977) and Root and MacLachlan (1978) discussed the stratigraphy and structure of the western end of the Hamburg klippe. Recently, Lash (1980a, b) and Kodama and Lash (1980) have described a plate tectonic model for the emplacement of the Hamburg klippe. GEOLOGIC FRAMEWORK
In the Kutztown quadrangle, the rocks of the Hamburg klippe are thrust to the north onto the Shochary Ridge sequence (Lyttle and Drake, 1979) of interbedded graywacke and shale of Middle to Late Ordovician age (Wright and others, 1978, 1979). The allochthonous rocks in the Hamburg quadrangle are in fault contact with the clastic Upper Ordovician(?) and Lower and Middle Silurian rocks of the Shawangunk Formation and Upper Ordovician and Lower Silurian(?) rocks of the Spitzenberg Conglomerate of Whitcomb and Engel (1934). These rocks mark the southern boundary of the Valley and Ridge in the area of study. The allochthonous rocks are bounded to the south by parautochthonous rocks of the Lehigh Valley sequence, the contact being the Kutztown thrust, a gently southeast-dipping fault. The Lehigh Valley sequence (MacLachlan, 1967) consists of the Allentown Dolomite, Beekmantown Group, Jacksonburg Limestone, and Martinsburg Formation (fig. 2). These rocks form the cover sequence of some of the nappes of the Reading Prong nappe megasystem of Drake (1978), a system of crystalline-cored,
GEOLOGIC FRAMEWORK
stacked nappes in the Great Valley of east-central Pennsylvania and western New Jersey. The Lehigh Valley sequence, therefore, is considered a parautochthonous sequence in the Alpine sense. TERMINOLOGY
Some mention of terminology employed in this paper should be made. Bouma sequences (fig1. 3) are used in a descriptive and nongenetic sense in discussions of the clastic rocks. The rock color chart (Goddard and others, 1948) is used to describe rock colors on fresh surfaces except where otherwise noted. Bedding parameters cited in the lithologic descriptions are from McKee and Weir (1953). STRATIGRAPHY AND SEDIMENTATION
Field mapping, which led to this paper, indicates that the allochthonous rocks at the eastern end of the Hamburg klippe occur in two tectonic slices or lithotectonic units (fig. IB). These slices are in fault contact and probably are bounded above by a thrust fault. The lower Greenwich slice, named for Greenwich Township in Berks County, Pa., is overlain by the Richmond slice, named for Richmond Township, Berks County, Pa. Rocks within the Richmond slice of the Hamburg klippe comprise a sequence of quartzose rocks, carbonate rocks, and black and green shale and mudstone that is herein named the Virginville Formation. The tectonically underlying, but younger, Greenwich slice consists primarily of graywacke and shale and lesser amounts of limestone, chert, and conglomerate and is here named the Windsor Township Formation. WINDSOR TOWNSHIP FORMATION
The Windsor Township Formation is here named for exposures along the east and west banks of Maiden Creek in Windsor Township, Berks County, between the towns of Dreibelbis and Kempton in the Kutztown and Hamburg 7%-minute quadrangles (fig. IB). The formation is predominantly a graywacke-shale unit that locally contains boulder conglomerate and slivers of limestone, chert, and variegated shale and mudstone. The Windsor Township Formation is divided into three mappable members, here named (1) Weisenberg, (2) Switzer Creek, and (3) Dreibelbis. The Weisenberg is predominantly a shale, siltstone, and mudstone unit, and the Switzer and Dreibelbis are graywacke-shale sequences. The Windsor Township Formation was mapped as "Lithotectonic Unit A" by Alterman (1972). She
demonstrated that rocks of the Hamburg klippe and the Martinsburg Formation are different and should not be treated as lithologic equivalents. Our detailed mapping substantiates Alterman's contention thatthe Windsor Township is a distinct lithotectonic and rockstratigraphic unit. Intraformational faulting and the lack of marker beds preclude an accurate thickness determination, but the unit has a minimum thickness of 6,215 m, calculated on the basis of cross sections constructed for the Kutztown quadrangle (Lash, in press). This thickness may be somewhat high because of internal deformation. WEISENBERG MEMBER
The rocks within the Weisenberg Member of the Windsor Township Formation consist of poorly cleaved to fissile, light-olive-gray (5F 5/2) to olive-gray (5F 3/2) and grayish-olive (10F4/2) shale, mudstone, and claystone to micaceous siltstone, containing local interbeds of medium-dark-gray (7V4) to dark-greenish-gray (5GY 4/1) slightly silicified shale and mudstone. Locally associated with the pelitic rocks are wellsorted, parallel to cross laminated pale-brown (5YR 5/2) and yellowish-gray (5F 7/2) thin-bedded siltstone, showing well-developed slump folds. The member also contains minor amounts of graywacke and chert. Polymict conglomerate deposits, here called the Werleys conglomerate and informally used to identify lenticular channel fill deposits in the Weisenberg Member, contain clasts of carbonate rock, silicified mudstone, and chert that are locally conspicuous. They are interpreted to be debris flows. The Werleys conglomerate is best exposed in outcrops about 1,070 m east of Werleys Corner in the Slatedale 7 1/2-minute quadrangle (fig. IB). The conglomerate is a poorly sorted, polymict lenticular channel fill that contains clasts of limestone, shale, and sparse volcanic rocks. Clasts are as much as 7.5 cm in diameter. Weathering of the limestone clasts gives the rock a characteristic pitted appearance. Although outcrops are sparse, Werleys conglomerate is abundant in the Topton and Slatedale quadrangles and, to a lesser extent, in the New Ringgold and Kutztown quadrangles. The Weisenberg Member is named for a series of exposures located about 1 km south of Weisenberg Church in the Slatedale quadrangle (fig. IB). It varies in thickness from one area to another, depending on the presence or absence of the graywacke members discussed above. The thickest sequence of the Weisenberg in the area studied is 1,740 m, an estimate based
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA 79°
77°
75°
41°-
Va lley
and
Ridge
20
I 20
I 30
30 I I 40
40
50 MILES I
I 50 KILOMETERS
EXPLANATION GREAT VALLEY ROCKS 1 Allochthonous rocks
Cambrian and Ordovician carbonate rocks and quartzite
Nx '
Proterozoic Y rocks
Martinsburg Formation
STRATIGRAPHY AND SEDIMENTOLOGY 75°45'
76° 00'
;+. '.+".'4 '
. .+ . *+ .. 4-
4
'
'
* .".+. . '
.4'
40°37'30"
0 Hamburg /reservoir Lenhartsville Breibelbts*
40°30
5 MILES 7 KILOMETERS
EXPLANATION Silurian and younger rocks
^^H Shochary Ridge sequence
Rocks of the Greenwich slice
T
Steeply dipping upthrust arrow on upper plate
Rocks of the Richmond slice
*
Low-a nglethrustfau It teeth on upper plate
Parautochthonous Lehigh Valley sequence (includes the Pen Argyl Member of the Martinsburg Formation)
FIGURE 1. Maps of eastern Pennsylvania. A. Map showing location of study area. B. Generalized geologic map of part of eastern Pennsylvania and western New Jersey. After Root and MacLachlan (1978). C. Generalized geologic map of the Kutztown and Hamburg 7%-minute quadrangles and surrounding areas. Geology of the Temple quadrangle modified from Wood and
~*~^
Tear fault Normal fault
MacLachlan (1978). Geology of the New Ringgold, New Tripoli, and Slatedale quadrangles from Lyttle and Drake (1979). Quadrangles are Orwigsburg, OR; New Ringgold, NR; New Tripoli, NT; Slatedale, SL; Auburn, AU; Hamburg, HB; Kutztown, KU; Topton, TO; Bernville, BE; Temple, TE; Fleetwood, FL; and Maxatawny, MA.
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
COMPLETE SEQUENCE
Martinsburg Formation
e petitic interval
Trentonian
d upper interval of parallel lamination c interval of current ripple lamination
Black River
o
>
o
Q GC O
b lower interval of parallel lamination
Chazyan a graded interval
Canadian ) graphitic shale and dark-yellowishorange (WYR 6/6) to pale-yellowish-orange (WYR 8/6) dolostone and calcisiltite are locally present. These sequences are, in some places, highly contorted and suggest some type of soft sediment deformation (fig. 24). Carbonate-clast conglomerates within the Moselem Member are generally thinner than those in the Onyx Cave Member. In addition, the average clastmatrix ratio is somewhat lower in the conglomerates of the Moselem than that in the Onyx Cave. Mudstone and shale, locally well cleaved, are the dominant rocks of the Moselem Member (fig. 25). Alterman (1972) noted the extreme variability of these rocks and attempted to divide them into mappable units. In the western part of the Kutztown quadrangle and the eastern part of the Hamburg quadrangle, the Moselem consists of thin- to thick-bedded, graphitic, grayish-black (7V2) mudstone and shale interbedded with dusky-brown (5YR 2/2) mudstone or claystone and locally ribbon limestone. Lenses rich in pyrite and as much as 18 cm long occur at places. The weathering of the pyrite gives the rock an orange-tan color. Other lithologies within the member include thin- to thickbedded, grayish-black (N2) dolostone interbedded with medium-dark-gray (7V4) to medium-gray (N5) mudstone and light-gray (N7) siliceous shale.
FIGURE 25. Well-bedded, dark-gray mudstone and shale of the Moselem Member. Exposed in a quarry approximately 915 m northwest of Virginville, Hamburg quadrangle. Hammer for scale.
In the western part of the Hamburg quadrangle, the dominant rock type is dark-gray (N3) to darkgreenish-gray (5G 4/1) siliceous argillite, which contains lenses and discontinuous beds of chert. Thinbedded, very light gray (MS) quartzite is interbedded with mudstone and shale locally. Pyrite nodules are extremely common in this rock and discolor it upon weathering. The stratigraphic complexity of the Moselem Member probably results from the intertonguing of the different shales, mudstones, and carbonate rocks. Carbonate rocks are not restricted to any specific shale type but seem to be more commonly associated with the darker shales and mudstones rather than with the green varieties. FIGURE 24. Contorted beds in interlaminated black shale and orange dolostone of the Moselem Member. Width of field is approximately 7 cm. Roadcut approximately 640 m north of Virginville, Hamburg quadrangle.
AGE AND CORRELATION
The age of the Virginville Formation is not known for certain. Recent conodont collections from the
22
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
Moselem and Onyx Cave Members (J. E. Repetski, written commun., 1978, 1979, 1980) suggest a Late Cambrian to late Early Ordovician (Arenigian) age (fig. 26). These rocks, therefore, are coeval with some of the bodies of chert, limestone, and variegated shale of the Windsor Township Formation. They are also coeval with parts of the Allentown Dolomite and Beekmantown Group of the Lehigh Valley sequence. The Virginville Formation cannot be correlated lithologically with any of the units of the Lehigh Valley sequence. Parts of the Virginville do resemble the Araby Formation and parts of the Frederick Limestone of the Frederick Valley, Md. (Reinhardt, 1974, 1977). More specifically, the Sacony Member resembles the Araby Formation. Reinhardt (1974, p. 7) described the Araby, the Antietam Quartzite of Jonas and Stose (1938). as a "buff to tan or green siliciclastic rock unit" in which "bedding is poorly defined."
FIGURE 26. Age relations between the Lehigh Valley sequence, the Windsor Township Formation, the Virginville Formation, and the Jutland klippe. Age data from Bergstrom and others (1972), Epstein and Berry (1973), Wright and others (1978), Perissoratis and others (1979), and J. E. Repetski (written commun., 1978, 1979, 1980).
The Onyx Cave Member is lithologically similar to the lower member of the Frederick Limestone, the Rocky Springs Station Member (Reinhardt, 1974, 1977) that conformably overlies the Araby Formation, whereas the Moselem Member has no lithologic counterparts in the Frederick Valley. Despite the strong lithologic comparisons of the Frederick Valley sequence and parts of the Virginville Formation, a direct biostratigraphic correlation is lacking. In fact, the limited amount of faunal data collected from the Virginville suggests that the rocks of the Virginville are younger than the rocks of the Frederick Valley sequence. ENVIRONMENT OF DEPOSITION
Stratigraphic and sedimentologic studies of the Virginville Formation indicate that the formation can be described as a slope and toe-of-slope deposit and probably part of an upward-shallowing sequence. Reinhardt (1974, 1977) envisioned the same paleoenvironment for the Araby Formation and the Frederick Limestone of Maryland. Study of the Sacony Member of the Virginville Formation suggests that the Sacony was deposited in a low-energy clastic basin below wave base, similar to the environment suggested by Reinhardt (1974) for the Araby. The strained quartz and minor, but conspicuous, amounts of microcline and plagioclase as framework constituents are indicative of derivation from a plutonic-metamorphic source, with some input from sedimentary rocks in the form of chert, and not from a volcanic source as suggested by Alterman (1972). Reinhardt (1977), on the basis of geochemical evidence, suggested a westerly cratonic source for the Araby. For reasons explained below, however, a southeasterly cratonic source is proposed for the Sacony Member. The transition from the siliciclastic rocks of the Sacony Member to the carbonate rocks of the Onyx Cave Member, a slope deposit, is abrupt. The Onyx Cave consists of thin-bedded ribbon limestones and thinly interlaminated black shale and dolostone suggestive of deposition in a quiet slope or toe-of-slope environment below wave base. The carbonate-clast conglomerates and thick, massive calcarenites indicate periodic influxes of material related, in part, to local slumping on the otherwise quiescent slope. Keith and Freidman (1977) have described a similar type of slope and toe-of-slope sequence from the Taconic region of New York. Mcllreath and James (1978) have developed two facies models for carbonate slopes on the basis of the rock record and modern environments. The models are (1) depositional margins typified by slopes that are gentle and decrease in inclination basinward and (2)
STRATIGRAPHY AND SEDIMENTOLOGY
23
bypass margins characterized by a margin atop a cliff clast conglomerates. or submarine escarpment. The main difference beThe random orientation of clasts, clasts "floating" tween the two models is that, in bypass margins, mate- in matrix, poor sorting, and lack of grading are sugrial is transported directly from the shallow-water gestive of some type of viscous debric flow mechanism shelf to a deepwater environment, thus bypassing a (Johnson, 1970; Cook and Taylor, 1977; Hampton, large part of the slope; in the depositional margin, 1972). In addition, plug flow (Johnson, 1970; Shanmuhowever, shallow-water material may not reach the gan and Benedict, 1978) is indicated by the clasts that middle or lower part of the slope because of the gentle project through the top of beds. inclination. In addition, the nature of the slope sediRecently, Krause and Oldershaw (1979) have dements are, in part, dependent upon whether the veloped a two-layer sediment gravity flow model to shallow-water margin is formed (1) by lime sands or explain the presence of turbidite caps on conglomerate (2) by reefs and associated reef material. The rock types beds. The few conglomerate beds of the Onyx Cave and sedimentary structures indicate that the rocks of Member that are capped by turbidites can be classithe Onyx Cave Member of the Virginville Formation fied as stratified disorganized beds and are interpreted were deposited on a depositional margin dominated by to result from deposition farther downslope than the lime and quartz sand. The lack of reef-derived clasts beds that lack the turbidite caps (Krause and Olderand fossils and the presence of well-rounded quartz shaw, 1979) or the disorganized beds. Hampton (1972) and carbonate grains support the depositional margin and Krause and Oldershaw (1979) attribute the stratimodel, as opposed to the bypass margin model. Other fied cap to turbidity currents that develop on top of the evidence includes slope-derived carbonate-clast con- main mass of sediment. Material in front of the flow is glomerates, massive to graded calcarenite, parallel- sheared backwards and on top of the moving mass and cross-laminated peloidal calcarenite, and slumped creating turbulence. ribbon limestone (fig. 27 and Mcllreath and James, The conglomerates probably result from transla1978). tional and rotational slides (Cook, 1979). The lack of Mcllreath and James (1978) pointed out that none the turbidite caps on the majority of conglomerates of the slope facies are mutually exclusive, and, as such, suggests that the sediment mass did not move far from the rocks of the Onyx Cave Member may have origi- its source. The local origin of the conglomerates is nated on a depositional margin characterized, in part, further supported by the clasts being of deepwater by reefs. In this case, the reef-derived material would rather than shallow-water carbonate rocks. The imnot have traveled far enough downslope to become portance of this relation was noted by Cook and Taylor incorporated in the rocks of the Onyx Cave or Moselem (1977). Thus, the carbonate-clast conglomerates probMembers. Nevertheless, the main body of evidence ably originated in a slumped interval in which the points to a margin characterized by lime and quartz sediment shear strength was exceeded, causing the sand and not reefs. mass to become mobile, break up, and become a debris Analysis of the Moselem Member suggests that flow. Although all clasts are locally derived, no conthe Moselem can be interpreted as a toe-of-slope facies glomerate could be traced backward to an obvious and probably a distal equivalent of the Onyx Cave slump or slide. The mechanism of deposition of the planar- to Member (fig. 27), although the Moselem is now in fault contact with the latter. In addition, the Moselem current-ripple-laminated black shale and yellowprobably interfingers with the Onyx Cave, as sug- orange dolostone and calcisiltite is not fully undergested by the presence of rock types common to each stood but may be the result of contour-following curunit. Local slumped horizons and carbonate-clast con- rents that redistributed previously deposited material. The carbonate rocks were apparently deposited glomerates in the Moselem attest to a lower slope or toe-of-slope environment, although movement can by a continuum of depositional mechanisms that were in operation on the slope. That is, the mechanisms occur on slopes as low as 4° (Lewis, 1971). The carbonate rocks of the Virginville Formation were operating together such as do dilute turbidity were deposited by several different depositional currents related to emplacement of massive grain mechanisms. Hemipelagic sedimentation and dilute flows, or in opposition, such as do contour currents turbidity currents probably resulted in deposition of reworking and redistributing detritus deposited by the ribbon limestone. The rhythmic sequences of lime dilute turbidity currents and debris flows. In summary, then, the Sacony Member of the Virmudstone and black shale are suggestive of deposition by turbidity currents (Wilson, 1969). The sequences ginville Formation was deposited in a low energy silimay be overbank levee deposits associated with the ciclastic basin below wave base. The transition of the emplacement of the thick calcarenites and carbonate- Sacony Member to the Onyx Cave Member, the slope
24
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
NW
ONYX CAVE AND MOSELEM MEMBERS
SACONY MEMBER
Ribbon limestone
Siliciclastic rocks
Graded to massive calcarenite
Shale and mudstone
Slumped hemipelagic rocks Limestone conglomerate liTTTii
Black and green hemipelagic mudstone and argillite
FIGURE 27. Facies diagram for the Sacony, Onyx Cave, and Moselem Members of the Virginville Formation. Stratigraphy is idealized and is in no way intended to imply specific horizon locations or thickness; the diagram is meant only to illustrate the proposed distribution of rock types.
facies, indicates the development and progradation of a carbonate shelf and slope. This slope is interpreted as a depositional rather than a bypass slope. PALEOGEOGRAPHY
The facing direction of the slope recorded by the rocks of the Virginville Formation is not known for
certain. Some information concerning the direction of slope can be obtained by analysis of slump folds in the Onyx Cave Member and tectonic folds along the fault separating the Onyx Cave and Sacony Members from the Moselem Member. The orientation of fold axes, axial planes, and bedding in slumped rocks has been used to determine
25
STRATIGRAPHY AND SEDIMENTOLOGY
gravity transport direction (Hansen, 1967; Corbett, 1973; Hall, 1973; Stone, 1976). In addition, fold axes and fold vergence have been used to determine movement directions in tectonic folds (Moore and Wheeler, 1978; Wheeler, 1978). All of these studies employed a method of fold analysis developed by Hansen (1967, 1971) in which the slip or slump movement plane and direction of movement within this plane, the slip line, of a slump or tectonic sheet could be obtained by plotting the axes and sense of vergence of asymmetric folds on a stereo net. The investigations cited above and numerous others have shown that this method is useful in paleogeographic reconstructions and, therefore, is suited to the present study. Excellent exposures of slump folds in the Moselem Member of the Virginville Formation can be found along the Reading Railroad grade approximately 1.2 km southwest of Shoemakersville in the Temple quadrangle (fig. 28). The absence of cleavage and joints that are generally related to tectonic folds of the area, the truncation or beveling of some of the folds by overlying undeformed beds, and the general restriction of deformation to a single bed are suggestive of a softsediment origin of the folds. Slip-line analysis of these folds (fig. 29) indicates that mass movement, probably
by translational sliding, occurred on slopes that were inclined to the northwest. Fault-related folds at the contact of the Moselem Member of the Onyx Cave and Sacony Members are characterized by overturned flexural-slip folds. These folds are always associated with an autoclastic melange that is restricted to the fault zone and thereby suggest a tectonic origin for the folds. Although a good amount of azimuthal scatter of the data occurs (fig. 30), analysis of the vergence of these folds indicates northwest transport of the upper plate containing the more "proximal" Onyx Cave Member over the more "distal" Moselem Member. In other words, the rocks of the Onyx Cave Member were deposited to the southeast of the rocks of the Moselem Member. In summation, analysis of the tectonic and slump folds in the Virginville Formation indicates that (1) extensive remobilization of semiconsolidated to unconsolidated sediments occurred by translational sliding on slopes that were inclined to the northwest and (2) the more proximal carbonate slope rocks (Onyx Cave Slip-line direction N. 30° W.
EXPLANATION
Pole to average bedding plane -» FIGURE 28. Soft-sediment folds in ribbon limestone of the Moselem Member. Folds verge to the northwest (right). The layer of limestone pull-aparts attests to the unconsolidated nature of some of the sediments. Exposure along the Reading Railroad grade approximately 1.2 km southwest of Shoemakersville, Temple quadrangle. Hammer for scale.
Pole to slip plane
FIGURE 29. Soft-sediment fold axes (dots), fold vergence (semicircular arrows), calculated slip plane (dashed girdle), and slip-line orientation of slump folds in the Moselem Member along the Reading Railroad grade 1.2 km southwest of Shoemakersville, Temple quadrangle.
26
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA Slip-line direction N. 18°W.
each characterized by a unique sedimentary history and, to a lesser extent, different structural styles. These terranes are the Valley and Ridge province in which the Upper Ordovician(?) to Lower Silurian Shawangunk Formation and younger rocks are exposed, the allochthonous Hamburg klippe consisting of the Greenwich and Richmond slices, and the parauthochthonous Lehigh Valley sequence of the Reading Prong nappe megasystem. The latter two are important to the present study, and their deformation is described below. ROCK FABRIC
FIGURE 30. Fold axes (dots), vergence (semicircular arrows), and slip-line orientation of tectonic folds related to thrusting along the fault separating the Onyx Cave and Sacony Members and the Moselem Member.
Member) were thrust to the northwest over their basinal equivalent (Moselem Member), indicating that the rocks of the Onyx Cave Member were deposited to the southeast of the rocks of the Moselem Member The rocks of the Virginville Formation may be a remnant of a sequence of carbonate slope and rise sediments that were deposited adjacent to a shallowwater carbonate shelf. This shelf may have formed on a topographic high or microcontinent to the southeast of North America. The rocks of the Virginville Formation may have been deposited on the northwestfacing bank of the microcontinent concomitant with shelf sedimentation on the southeast-facing slope of the North America craton. Thus, the Virginville Formation may represent a rock-stratigraphic, if not timestratigraphic, equivalent of the Frederick Valley sequence, which was deposited on a southeast-facing slope (Reinhardt, 1974, 1977). STRUCTURE
The part of the central Appalachians of the present study is divisible into three structural terranes,
The rocks of the Lehigh Valley sequence and of the Hamburg klippe were folded during three main phases. The first fold phase (Fj) is marked by east-northeasttrending isoclinal to open, asymmetric to symmetric folds of various wavelengths and amplitudes. Axial planar slaty cleavage (SJ is well developed in the rocks of the Lehigh Valley sequence and locally in the less competent rocks of the Richmond slice of the Hamburg klippe. In contrast, the rocks of the Greenwich slice of the Hamburg klippe lack the well-developed slaty cleavage and are, instead, characterized by a "scaly" type of cleavage that is discussed in more detail on page 27. The second fold phase (F 2) is characterized by northeast-trending, parallel, symmetric to asymmetric, generally open folds of outcrop scale to small crenulations. A well-developed fracture and strain-slip or crenulation cleavage (S 2) has formed in response to folding. In some areas, earlier F 2 parallel folds were flattened, with resultant thickening of the fold hinges and development of a south-dipping fracture cleavage that fans the folds. F 2 structures are well developed in all rocks of the eastern end of the Hamburg klippe. The latest fold phase (F3) is only locally developed. It is characterized by east-northeast-trending kink folds, small (meter-scale) open folds, and crenulations. Detailed structural analysis indicates that the rocks of the Lehigh Valley sequence have been folded by all three fold phases. In contrast, ongoing studies of the Upper Ordovician to Silurian rocks of the Valley and Ridge northeast of Hamburg indicate that only the F 2 and F 3 fold phases are recorded by these rocks. This suggests, then, that the F] fold phase is related to a pre-Silurian (Taconic?) erogenic event and the F2 and F3 folds are related to post-Silurian (Alleghanian) events. Additional evidence for the pre-Silurian fold
27
STRUCTURE
phase is (1) the apparent unconformable relations that exist between the steeply dipping allochthonous rocks and the overlying near-horizontal Upper Ordovician Spitzenberg Conglomerate (Whitcomb and Engel, 1934) north of Lenhartsville and (2) the exposed angular unconformity and de'collement between the Shawangunk Formation and the Hamburg klippe at Schuylkill gap. FAULTS
Low-angle thrust faults dominate the complex structure of the eastern part of the Hamburg klippe. The parautochthonous Lehigh Valley sequence is separated from the Hamburg klippe by the Kutztown thrust (fig. IB), a gently southeast-dipping thrust fault that has cut numerous early faults within the Lehigh Valley sequence. MacLachlan (1979), working in the Temple and Fleetwood quadrangles (fig. IB), referred to the Kutztown thrust as the Leinbachs thrust and maintained that it dips gently to the north with the Lehigh Valley sequence thrust beneath the klippe. However, the conspicuous occurrence of northdipping beds located along the trace of the fault about 3 km southeast of Virginville suggests that the Lehigh Valley sequence moved .to the northwest over the allochthon, thereby dragging rocks that originally dipped south into north-dipping attitudes. The evidence suggests that the fault dips south in this area. The Hamburg klippe is separated from the rocks of the Shochary Ridge sequence by the Kistler Valley fault, a steep southeast-dipping upthrust (Lyttle and Drake, 1979). On the basis of paleontologic evidence, Wright and others (1978, 1979) have questioned the presence of this fault and have, instead, suggested a depositional contact between the two sequences of rocks. This interpretation cannot explain the faultrelated deformation along the Kistler Valley fault (Lyttle and Drake, 1979). In addition, mapping in the Shawangunk Formation north of Hamburg (Lash, 1980b) suggests that the Kistler Valley fault continues west into the Silurian clastic rocks north of Lenhartsville where movement on the fault has resulted in bedding discordancies in the rocks. The fault separating the Richmond slice from the Greenwich slice of the Hamburg klippe is not exposed but is inferred from both the highly contrasting styles of deformation (discussed on p. 26) and the vastly different sedimentologic characteristics of the two slices. Additional evidence in support of the thrust contact of the two allochthonous slices can be found in exposures along the Reading Railroad grade 1.5 km
west of Shoemakersville in the Hamburg quadrangle. At this locality, slices of Middle Ordovician (Nemagraptiis gracilis graptolite zone) rocks of the Windsor Township Formation are found associated with larger slices of Lower Ordovician (Arenigian) ribbon limestones of the Moselem Member of the Virginville Formation. The tectonic slices of the Windsor Township rocks apparently represent slices of underlying rock detached and dragged (technically mixed?) into present position during emplacement of the Richmond slice much in the same way as the tectonic slices of the Stockbridge Formation of the Taconic Range were dragged along the sole of the allochthonous Everett Formation (Zen and Ratcliffe, 1966). STYLE OF DEFORMATION
As described on page 26, the rocks of the Windsor Township Formation in the Greenwich slice lack the well-developed slaty cleavage of the equally incompetent Martinsburg Formation and Jacksonburg Limestone. Instead, they exhibit a "scaly" cleavage characterized by a poorly developed fracture cleavage that consists of a group of intersecting fracture planes. Cowan (1974) has referred to this type of cleavage as a "penetrative shear-fracture fabric" apparently produced by localized losses of resistance at geologically high strain rates (Raymond, 1975a), for example, strain rates typical of subduction complexes. Without going into the details of melange nomenclature (for discussion see Berkland and others, 1972; Raymond, 1975a, b; Beutner, 1975), Cowan's (1974) term "tectonic melange" will be used to describe the rocks of the Greenwich slice. The term "tectonic melange" as defined by Cowan emphasizes that the melange records a deformation without regard to types of inclusions or even the presence of inclusions. The main internal feature typical of tectonic melanges as described by Cowan is the pervasive, mesoscopic shear fracture. In the Greenwich slice, the shear fractures in the less competent argillaceous rocks result in a scaly appearance in which individual chips are polished and slickensided. Because of relative differences in competency, the rocks take on a very chaotic appearance upon deformation and are characterized by lentils or phacoids of more competent rocks embedded in a sheared matrix of less competent material (fig. 31). This phenomenon is best illustrated in the rocks of the Greenwich slice by zones of intense strain, here referred to as deformation zones, marked by small pull-apart (fig. 32) and small- to large-scale pinch-and-swell (fig. 33) structures.
28
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
FIGURE 31. Large boudin or phacoid of graywacke embedded in a sheared pelitic matrix. Width of boudin is approximately 2.5 m. Roadcut along: Route 143 approximately 360 m west of Albany. Kutztown quadrangle.
Outside the deformation zones, the less competent mudstone shows the penetrative shear-fracture fabric, and individual sandstone beds are essentially undeformed, although some beds are displaced along fractures at low angles to bedding. Disruption continues to increase in intensity toward the deformation zone until only isolated phacoids of beds surrounded by a completely sheared argillite matrix remain. This progressive style of deformation has been described from the Garzas tectonic melange of the Franciscan Complex by Cowan (1974). In addition, Hamilton (1979) noted a back-and-forth gradation in disruption, with increasing shearing into a fully developed melange characterized by abundant lenses and blocks, on Timor Island. In his study of tectonic melanges. Cowan (1974) discussed the importance of determining the parentage of the melange. He noted that, although the position of an inclusion may now be relatable to surfaces of shear, the presence of the inclusion itself may have resulted from chaotic submarine slumps and not from tectonic mixing. Within the Windsor Township For-
FIGURE 32. Small graywacke boudin embedded in a highly sheared pelitic matrix. Note scaly cleavage or "penetrative shearfracture fabric" (Cowan, 1974) of the pelitic matrix. A late subhorizontal northwest- (right-) dipping fracture cleavage cuts both matrix and boudin. Roadcut along Route 143 approximately 360 m west of Albany, Kutztown quadrangle. Lens cover for scale.
mation, "exotic" inclusions are represented by the Lower Ordovician chert, limestone, and variegated shale units. These inclusions were probably emplaced as subaqueous gravity slides into Middle Ordovician flysch prior to melange deformation of these rocks. This distinction is a very important point in the final tectonic synthesis. The development of the tectonic melange fabric in the rocks of the Greenwich slice occurred prior to F! folding of the allochthon and parautochthon. This relation is illustrated in the Albany area and the area south of Dreibelbis, where individual deformation zones have been mapped around F, folds. As noted on page 14, the Greenwich slice is lithologically similar to the Summerdale allochthon of Root and MacLachlan (1978) at the western end of the Hamburg klippe. This correlation is further supported
STRUCTURE
FIGURE 33. Pinch-and-swell structure. The more competent graywacke has deformed brittlely, whereas the less competent pelite has flowed around the graywacke. Roadcut along Route 143, approximately 360 m west of Albany, Kutztown quadrangle. Notebook in lower left corner for scale.
by the similar deformational styles of the two allochthons. Root and MacLachlan (1978) noted that rocks of the Summerdale allochthon have characteristics of a "highly sheared melange." The implication of the correlation from the eastern end of the Hamburg klippe to the western end is that the lower lithotectonic units of the Hamburg klippe are characterized by a slice or slices of chert, limestone, and variegated shale. The chert, limestone, and variegated shale slices are tectonically overlain by a highly sheared graywackeshale slice that contains olistoliths of rocks similar to that in the underlying slice. These slices are in turn tectonically overlain by slices such as the Richmond slice. PLATE TECTONIC IMPLICATIONS
Many plate tectonic models have been proposed to explain the complex geology of the Appalachian oro-
29
gen from Newfoundland to Georgia (Bird and Dewey, 1970; Glover and Sinha, 1973; St. Julien and Hubert, 1975; Thomas, 1977; Hatcher, 1978; Osberg, 1978; Schenk, 1978). All of these workers agree upon the closing of a proto-Atlantic ocean and associated subduction and obduction that resulted in the Taconian and Acadian orogenies. From the combined structural, sedimentologic, and stratigraphic data obtained in this study, a conceptual plate tectonic model for the early Paleozoic of this part of the central Appalachians is described below. The proposed tectonic model (fig. 34) involves the aborted subduction of the North American Continent beneath a southeastern microcontinent. Tectonic models based on the abortive subduction of the continental margin have been suggested by other geologists for the northern and maritime Appalachians (Chappie, 1973; McBride, 1976; Hiscott, 1978; Stanley and Ratcliffe, 1979). Late Proterozoic rifting in response to opening of the proto-Atlantic ocean resulted in the formation of a marginal basin separating the North American Continent from a microcontinent to the southeast (fig. 34A). The rifted blocks or microcontinents may be represented by such Precambrian complexes as the Fordham Gneiss terrane, Baltimore Gneiss terrane, Pine Mountains, eastern Great Smokey Mountains, and the Sauratown Mountains (Rankin, 1975; Thomas, 1977). The Early Cambrian of the Appalachians was characterized by the formation of a stable carbonate platform on both the North American Continent (Rodgers, 1968) and the microcontinent (Thomas, 1977) (fig. 34^4). Development of the shelf on the North American craton is evidenced by the Frederick Valley sequence of Maryland interpreted by Reinhardt (1974, 1977) to be part of the southeast-prograding carbonate shelf. In addition, the Cambrian and Ordovician carbonate rocks of the Pennsylvania Great Valley attest to the vast amount of shallow-water carbonate sedimentation during the lower Paleozoic on the North American Continent. Shallow-water shelf sediments on the microcontinent may be represented by such sequences as the Setters Formation and Cockeysville Marble associated with the Baltimore Gneiss or the lower Inwood Marble associated with the Fordham Gneiss. Carbonate rocks of the Virginville Formation of the present study were deposited on a northwest-facing bank and record maturation of a stable carbonate platform to the southeast, probably a microcontinent. Shelf deposition and coeval deepwater sedimentation continued into Ordovician time. Some units of the Windsor Township Formation, variegated shale and mudstone and chert and limestone, represent deepwater pelagic sediments deposited concomitant with
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
30
A. Proterozoic Z to Early Cambrian. Rifting resulted in the separation of North America and a microplate located to the southeast, perhaps, represented by the Baltimore Gneiss terrane (Thomas, 1977). The separating basin was not very extensive and was probably underlain by thinned continental crust as rifting never progressed beyond the intracontinental phase and the major rift center formed to the east of the microplate. The lack
of ophiolite debris in the flysch sequences of the Appalachian Great Valley in Pennsylvania precludes the development of extensive oceanic crust. Rocks of the Richmond slice of the Hamburg klippe were deposited on the northwest-facing lower slope of a passive continental margin on the microplate while coeval carbonate shelf deposits (Allentown Dolomite, Conococheague Group, for example) were deposited on the shelf of the North American craton.
SE
NW SEA LEVEL
+
+
-North American shelf
+'
+
+
+
+
+ + + + + +North American craton
+
+
+
+
Early
+ + + + + 4+ + Rifted microplate 4+ + + + + + + + + x + + 4+ + + + + +
+ +
+
Cambrian
+
B
SEA LEVEL Richmond slice-_ ^
Longitudinally derived Lower (?) to Middle
Ordovician clastic sediments and Lower to Middle (?) Ordovician pelagic sediments
'"*"*
+
4-
+
4-
+
4-
+
+
+
+
+
+
4-
+
4
Late Cambrian 444 + + to Middle + + + + Ordovician +
Peripheral bulge
+
+
+
+
+
EXPLANATION 4 4-
4
Gneiss, granite
Shale
Limestone
Shaley limestone
Sandstone
B. Late Cambrian to Middle Ordovician. Subduction was probably initiated by Early Ordovician time. Red and green shale of Early Ordovician age in the Greenwich slice reflects the uplift of a terrigenous source area. This uplift may record a collision at the New York Promontory that resulted in the longitudinal
dispersal of pelagite in Early Ordovician time followed by coarsegrained clastic sediment in Middle Ordovician time that was shed longitudinally from an uplifted area to the northeast. These clastic rocks were deposited as abyssal plain turbidites (Stewart, 1976; Dickinson, 1982).
FIGURE 34. PLATE TECTONIC MODEL FOR THE HAMBURG KLIPPE
31
PLATE TECTONIC IMPLICATIONS
C. Blackriveran. As the North American plate approached the trench, it was flexed upward resulting in rapid uplift and erosion of the shelf carbonates at the southeastern margin of the North American craton. Clastic sediments of the Greenwich NW
Lehigh Valley sequence
slice continued to be supplied from a northeasterly source. These sediments (clastic rocks stratigraphically overlying older pelagic sediments) were accreted onto the hanging wall of the trench. The Greenwich slice is part of this accretionary prism. SE
.Black River hiatus
SEA LEVEL Accretionary prism (Greenwich slice)
Blackriveran
Black River hiatus
Lehigh Valley sequence
Lehigh Valley sequence^
SEA LEVEL Martinsburg Formation
. , Greenwich
RocklandianKirkfieldian
D. Rocklandian-Kirkfieldian. Rocklandian to Kirkfieldian time was characterized by deposition of the Jacksonburg Limestone during rapid basin subsidence. This concept is supported by the lithostratigraphic changes from the "cement-limestone" facies to the "cement-rock" facies of the Jacksonburg and by the transition from the shallow, warm-water North Atlantic province conodont fauna at the base of the Jacksonburg to the deep, cold-water North Atlantic fauna at its top. The basin probably formed by normal faulting in response to "trench suction" (Elsasser, 1971) rather than by tectonic loading at the continen-
tal margin. Deposition of the clastic rocks of the Martinsburg Formation was probably initiated at the end of Kirkfieldian time (all contacts are probably time transgressive). By this time, subduction had ended, and the closure of the Jacksonburg-Martinsburg basin was probably accomplished by reactivation of the early formed normal faults into thrust faults. The nappes of the Pennsylvania Great Valley and New Jersey were probably formed in this manner. After the Hamburg klippe was emplaced, it became a passive rider on the stacked nappe sequence. At this time of extreme compression, the Richmond slice was emplaced onto the Greenwich slice.
FIGURE 34.-PLATE TECTONIC MODEL FOR THE HAMBURG KLIPPE-Continued
32
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
E. Edenian. By Edenian time, the uplifted area related to the proposed collision at the New York Promontory had migrated laterally and closer to the Martinsburg basin of southeastern Pennsylvania and was now supplying the coarse clastic sediment of the Ramseyburg Member of the Martinsburg Forma-
tion. This uplift occurred as the tectonic wave was built up by the accretion of basement rocks and overlying miogeoclinal sequences (nappes). The nappes continued to move by reactivation of the early formed normal faults into thrust faults, and the basin continued to close. SE
NW Hamburg klippe ^ Black River hiatus
SEA LEVEL
+ -l-
Greenwich .slice ..Richmond _/ / slice
North American flank
Edenian (C. Spiniferus)
Lehigh ValleyM+ + + 4.4. sequence 4-4-4-4-4.4.4.4. Lehigh Valle 4.4-4.4-4. sequence
4- Crystalline-cored nappes 4
Hamburg klippe Richmond slice
SEA LEVEL
4-
4-
4-
Early Maysviliian (Late C. Spiniferus)
+
4+ 44- /+ 4. Muddy turbidites (Pen Argyi 4. 4. + Member of Martinsburg 44- Lehigh Valley 44- Formation) 4-4-4sequence '+ 4-4-4-4-4-4-4-4-
+
+ +
+
+ +
+
+ +
+
+ +
4-
+
+ +
+~~~^f~---j^__+'~'^f~ +
+
Crystalline-cored nappes
+"+ T -f -±____+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 4-+__^ J =t 1= i + ±__ + + + + + + + + + +^-~~~~~~~~~^ + + + + + + + + + + + + + + + + + + + + + +_
F. Early Maysviliian. By Maysviliian time, tectonic movement had slowed significantly. The source area was now lowered, and the more compositionally mature clastic rocks of the Pen Argyl Member of the Martinsburg Formation were deposited. The
+
+
+
+
Pen Argyl contains structures typical of muddy, slow-moving, low concentrate turbidity current deposits, as described by Stow and Shanmugam (1980), that suggest the progradation of a muddy shoreline.
FIGURE 34.-PLATE TECTONIC MODEL FOR THE HAMBURG KLIPPE-Continued
PLATE TECTONIC IMPLICATIONS
Early Ordovician shallow-water carbonate rocks of the Beekmantown Group. Closing of the marginal basin was accomplished by southeast-dipping subduction beneath the microcontinent. Subduction, which was probably initiated in early Early Ordovician time, resulted in the foundering or depressing of the carbonate shelf (fig. 345). Evidence for foundering of the lower Paleozoic North American shelf has been documented in Newfoundland by Stevens (1970) and also may be documented in rocks of the Lebanon Valley sequence of the central Appalachian Great Valley. MacLachlan and others (1975) noted that, in the Lebanon Valley west of Womelsdorf, Pa., the uppermost Ontelaunee Formation is overlain conformably or paraconformably by the Annville limestone, a high-calcium limestone unit, of definite Chazyan age. The change from almost pure dolomite of the Ontelaunee to the high-calcium limestone of the Annville may represent a transgressive period that resulted from foundering of the shelf. Therefore, progradation of the shelf seems to have continued until at least Whiterockian time and probably into Chazyan time. Progradation was followed by a period of transgression that resulted from foundering of the shelf in response to subduction and by deposition of rocks such as the high-calcium Annville Formation. The style of deformation of the Greenwich slice of the present study and of the Summerdale allochthon of the western end of the klippe suggests that these rocks were deformed in a subduction complex and, therefore, represent trench sediments (fig. 345). Paleogeographic reconstructions based upon the stacking order of thrust sheets have been documented from numerous areas (Williams, 1975; Elliot, 1976) and may be applied to the rocks of the Hamburg klippe. If northwest transport during closing of the marginal basin can be assumed, the stacking order of the slices in the western end of the Hamburg klippe suggests that the abyssal sediments of the Enola allochthon were northwest of the rocks of the Summerdale allochthon and, by correlation, the Greenwich slice. In other words, the abyssal sediments were deposited to the northwest of the subduction zone. Sparse directional paleocurrent data evince both northwesterly and southeasterly sources, but, as stated earlier, analysis of slump folds supports a predominantly southeasterly source, possibly the microcontinent. Despite this evidence, some of the detritus may represent second-cycle sediments originally deposited as continental slopes and rise sediments adjacent to the southeast-facing shelf. As subduction continued, the older abyssal sediments, represented by the bodies of red and green shale and accompanying rocks, became detached from
33
the underlying plate and slid into the trench. Moore (1975) maintains that physical properties such as density and yield strength of clastic sediments (that is, hemipelagic and pelagic mud and red and brown clays) and the underlying biogenic sedimentary unit diverge with time; however, the same physical properties of the biogenic unit and the underlying upper basaltic layer of oceanic crust converge. This difference is significant in that the low density of the clastic sediments and resultant relative buoyancy would tend to favor the incorporation of these sediments into the trench sediments at shallow levels, presumably by slumping or sliding along a de'collement separating the physically contrasting units (Moore, 1975). As the abyssal sediments approached the trench, the more buoyant red and green mudstone and shale units became detached and were incorporated into the shallow levels of the trench sediments in a manner similar to that described above. The great amount of soft sediment deformation in some of these units attests to slumping and sliding during emplacement. Following emplacement of these units, the rocks were severely sheared and strained resulting in the tectonic melange style of deformation. Root and MacLachlan (1978) maintain that the Enola and Summerdale allochthons were closely associated and were likely emplaced together from the same general source. Their assertion appears likely in that during subduction a large mass of pelagic sediments represented by the Enola allochthon became detached from the underlying plate along the de'collement separating the younger clastic sediments from the underlying units. This slice was accreted to the trench sediments, and the entire mass moved as a unit. It is possible, but difficult to prove at the present level of erosion, that detachment and accretion occurred several times prior to emplacement of the klippe onto the shelf. The majority of turbidite and grain flow deposits, as well as the olistoliths of pelagic and hemipelagic sediments, were emplaced during Chazyan time, essentially concomitant with foundering of the shelf. As the North American craton approached the trench, the inability of the buoyant continental crust to subduct beneath the microcontinent resulted in uplift of the continental margin. Erosion of the shelf and continental margin rocks accompanied uplift and is evidenced by the Black River hiatus in the Lehigh Valley sequence (see fig. 2) and the Lebanon Valley sequence that is somewhat different from the tectonically lower Lehigh Valley sequence (MacLachlan and others, 1975). The lack of a well-defined Black River hiatus in the Cumberland Valley sequence (Root, 1977, fig. 2), the tectonically lowest and probably the least
34
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
traveled sequence in the Pennsylvania Great Valley, indicates that the area of the shelf defined by the carbonate rocks of this sequence was not uplifted but, instead, remained submerged and developed an almost uninterrupted stratigraphic sequence. Therefore, the farther northwest one moves from the tectonic welt or arch, the smaller the unconformity becomes, until conformable or paraconformable relations are realized. Uplift and erosion of rocks of the Beekmantown Group are evidenced by the presence of dolomite clasts, presumably derived from the Ontelaunee Formation of the Beekmantown Group, within the lower unit, the "cement limestone" facies, of the Jacksonburg Limestone in New Jersey (Drake, 1969) and the Hershey Limestone, a unit which closely resembles the upper "cement rock" facies of the Jacksonburg (MacLachlan, 1967), near Myerstown, Pa. (MacLachlan and others, 1975). The presence of the basal calcirudite unit containing clasts of Beekmantown-type dolomite indicates that uplift of the continental margin was in progress by Rocklandian time, concomitant with deposition of the Jacksonburg Limestone. The change from the relatively shallow-water "cement limestone" facies to the deeper water "cement rock" facies of the Jacksonburg Limestone of the Lehigh Valley sequence indicates that the basin continued to deepen during Rocklandian and Kirkfieldian time. Deposition of the deepwater psammitic-pelitic Martinsburg Shale followed. The inability of the North American Continent to subduct beneath the microcontinent retarded and eventually stopped subduction and locked the two plates. As stress built up along the contact of the two plates, detachment of wedges or slabs of basement rock and associated miogeoclinal rocks occurred (fig. 34Z>). The detached basement represents the cores of the carbonate nappes of the Pennsylvania Great Valley. Because the Jacksonburg Limestone was involved in nappe formation (Drake, 1978), detachment and imbrication probably started somewhat after deposition of the Jacksonburg, possibly in Late Blackriveran to Kirkfieldian time. The technically telescoped and thickened continental crust resulted from large-scale underthrusting and imbrication of slabs or wedges of cratonic basement on easterly inclined faults or ductile shear zones (fig. 34Z>). Detachment of basement wedges during collisional tectonics has been described from the northern Appalachians by Stanley and Ratcliffe (1979) and Ratcliffe (1979). At this time of extreme telescoping, the Richmond slice (consisting of carbonate-slope rocks of the eastern microcontinent) probably was emplaced upon the Greenwich slice, which had become
part of a massive wedge of basement, miogeoclinal, and eugeoclinal rocks. Following the aborted subduction of the continental margin in late Blackriveran time, southeast-dipping subduction may have started to the southeast of the microcontinent. The continued southeast-dipping subduction probably contributed to the thrust emplacement of the Richmond slice and to further telescoping of the North American continental margin. The amount of time required to close the marginal basin that separated the North American Continent from the microcontinent as determined by the age of the trench sediments of the Windsor Township Formation is approximately two graptolite zones, an amount of time equal to that suggested by Chappie (1973) for a similar model for the northern Appalachians. Continued convergence drove the wedge of accreted rocks up the basal slope that had been created by subduction of the North American Continent. Elliot (1976) and Wiltscko (1979) maintain that thrust sheets can move up basal slope as long as sufficient surface slope has developed within the thrust sheet itself. The mass will move in the downdip direction of the surface slope even if this means moving up basal slope. Once the wedge of basement, miogeoclinal, and eugeoclinal rocks built up sufficient surface slope, probably by shortening in the back of the wedge, it was able to move up the basal slope. The massive wedge of thickened continental crust and rocks of the Hamburg klippe continued to move throughout late Middle Ordovician time and caused further depression of the shelf. Rocks of the Martinsburg Formation were deposited in a clastic wedge in a foredeep in front of the wedge of accreted rocks (fig. 34Z), E). The entire Martinsburg appears to have been involved in nappe formation to some extent, which suggests that the nappes continued to "form" into Maysvillian time (see fig. 34F). The nappes continued to develop by movement along thrust faults separating each of the basement-cored nappes. The Hamburg klippe may have become detached from the underlying nappes and may have slid in advance of them into the Martinsburg basin (fig. 34#). The mixed allochthonous-autochthonous terrane in the vicinity of Harrisburg, Pa., attests to the subaqueous gravity sliding mechanism for the emplacement of the Hamburg klippe into the Martinsburg basin. The age of the final emplacement of the klippe is not known. The matrix of the wildflysch at the western end of the klippe has not been dated, and there is no wildflysch at the eastern end despite previous reports (Alterman, 1972). If there were wildflysch at the eastern end, it has been removed by faulting or is not exposed at the
35
PLATE TECTONIC IMPLICATIONS
present level of erosion. Wright and Stevens (1978) suggest that the allochthons were emplaced during the lower Diplograptus multidens graptolite zone. This emplacement seems impossible if the minimum age of the Jacksonburg Limestone is early Rocklandian. Emplacement of the allochthons into the Martinsburg basin during lower Diplograptus multidens time would require uplift of the continental margin, detachment of basement and resultant nappe formation, and deposition of the Jacksonburg Limestone and a certain thickness of rocks of the Martinsburg Formation within a very short time. A more likely time of emplacement by analogy with the emplacement of the Normanskill Formation of New York State (Rickard and Fisher, 1973) and the Taconic klippe of New England (Zen, 1967) is the 0. Reudemanni graptolite zone or late Sherman Falls, although direct evidence is lacking. The true age of the final emplacement of the Hamburg klippe will not be known until datable fossils are obtained from wildflysch associated with the allochthons. Finally, as motion of the two plates slowed and eventually stopped, post-Maysvillian slow heating and uplift resulted in the unconformity separating the Taconides of the Great Valley and the Alleghanides of the Valley and Ridge (Drake, 1980). Late Alleghanian thrusting, such as that described by Root (1977) and Root and MacLachlan (1978), from the western end of the klippe and the Kutztown thrust of the eastern end continues to move the nappes of the Reading Prong nappe megasystem to the northwest over the klippe rocks. A modern analogy of the proposed model exists in the vicinity of Timor Island north of Australia. Timor Island, located in the outer Banda Arc (Hamilton, 1977, 1979; Von der Borch, 1979), consists of a chaotic complex of highly deformed and imbricated rocks and was interpreted to be a tectonic melange by Fitch and Hamilton (1974) and Hamilton (1979). Seismic profiles of the island indicate a wedge-shaped morphology (Von der Borch, 1979) that thins to the south. Deep-sea drilling in the Timor Trough (Hamilton, 1977) south of Timor shows that shallow-water carbonate rocks of the Australian shelf underlie the trench fill and indicates foundering of the shelf. Foundering of the Australian shelf and attendant block faulting has been attributed to subduction of the continental margin beneath the Banda Arc (Crostella, 1977; Hamilton, 1979). The chaotic rocks of Timor Island, therefore, are considered to be part of an accretionary prism of trench, shelf, and crystalline rocks that moved with the upper plate (Hamilton, 1979) during subduction of the Australian shelf and basement.
Although obvious differences exist, the proposal model and the present-day tectonics of the TimorTanimbar area are grossly similar. The melange rocks of the Greenwich slice are similar in style of deformation and stratigraphy to the melange on Timor Island. In addition, the crystalline cores of the nappes of the Reading Prong nappe megasystem are represented on Timor Island by slabs of what appear to be Australian basement. Future research on the rocks of the Great Valley will undoubtedly clarify certain points of the proposed model and help to refine our ideas of the early Paleozoic plate tectonics of the central Appalachians. CONCLUSIONS
Taconic-type klippen such as the well-known examples from the northern and maritime Appalachians also have been described from the central Appalachians. The Hamburg klippe of Stose (1946) constitutes one such body in the Great Valley of eastern Pennsylvania. The eastern end of the allochthon was mapped in the Kutztown and Hamburg 7%-minute quadrangles and surrounding areas. This mapping has led to the recognition of two stratigraphically and structurally different slices. The lower Greenwich slice consists of rocks of the Windsor Township Formation (6,215 m), a flysch sequence that contains various sized bodies of chert, limestone, and variegated shale and boulder conglomerate. The rocks of the Windsor Township are subdivided into three members: (1) the Dreibelbis, a sequence of interbedded graywacke and gray-green shale, siltstone, and mudstone, (2) the Weisenberg, a gray-green shale unit that contains local lenses of polymict conglomerate, and (3) the Switzer Creek, a conglomeratic graywacke-gray-green-shale unit. The lenticular bodies of chert, limestone, and variegated shale and mudstone are not restricted to a particular member but occur throughout the formation and, therefore, have little regional stratigraphic significance. Graptolites collected from the flysch sequence of the Windsor Township yield Middle Ordovician (Nemagraptus gracilis zone) ages, whereas conodonts from the carbonate rocks of the bodies of red shale and associated rocks yield Early Ordovician (Arenigian) ages. Therefore, a definite age discrepancy exists between the two types of rock within the Windsor Township Formation. Sedimentologic analysis of the flyschoid rocks of the Windsor Township Formation suggests that the rocks can be classified into one of four turbidite facies described by Mutti (1977) in his study of a flysch
36
RICHMOND AND GREENWICH SLICES OF THE HAMBURG KLIPPE IN EASTERN PENNSYLVANIA
sequence from the Spanish Pyrenees. These are (1) the channel-margin facies characterized by thick, massive (T a, T a_e partial Bouma sequences) graywacke beds locally interbedded with the T c_e partial Bouma sequence and very thick intervals of siltstone and mudstone, (2) the channel-axis facies characterized by monotonous sequences of thick, massive (T a, T a e) graywacke sandstone, (3) the interchannel facies characterized by sequences of bundles of T c_e partial Bouma sequences intercalated with mudstone and siltstone, and (4) levee and overbank deposits characterized by mudstone and shale, locally slumped. The Dreibelbis Member contains rocks of the channelmargin, channel-axis, and interchannel facies, whereas most rocks of the Switzer Creek Member appear to be typical of the channel-axis facies. The Weisenberg Member appears to be a good example of levee and overbank mudstone and siltstone deposits. These rocks were probably deposited in the middle-fan area of a large submarine fan from a source to the southeast. The activity of the channels in the fan is evidenced by the intercalated nature of channel-axis and interchannel facies rocks. This sediment mixing suggests that the channels were meandering freely rather than being incised into the fan mud as would be expected in the upper parts of the fan. The older lenticular bodies of chert, deepwater limestone, and variegated shale were apparently deposited in an abyssal environment that received periodic influxes of terrigenous material. They were later emplaced into the younger fan deposits. The upper Richmond slice contains rocks of the Virginville Formation (565 m thick) and is divided into three members. (1) The Sacony (245 m thick) consists of olive-green siltstone and shale. (2) The Onyx Cave (90 m thick) consists of massive to laminated calcarenite, ribbon limestone, massive carbonate-clast conglomerate, and interlaminated black shale and orange dolostone. (3) The Moselem (230 m thick) consists of well-cleaved argillite and mudstone and lesser proportions of ribbon limestone and carbonate-clast conglomerate. The limited amount of faunal evidence from these rocks suggests a Late Cambrian to Early Ordovician age. The sedimentology and stratigraphy of these rocks suggest that (1) they were deposited low on a northwest-facing depositional slope and (2) the Moselem Member is a distal equivalent of the Onyx Cave Member. The data suggest that the rocks of the Virginville were deposited on a northwest-dipping depositional slope, probably adjacent to a shallowwater carbonate shelf to the southeast. Subsequent thrusting juxtaposed the Sacony and Onyx Cave on top of the Moselem. The rocks of the Hamburg klippe have been deformed by three phases of folding. In addition, the
Greenwich slice illustrates the effects of an earlier phase of deformation not present in the rocks of the Richmond slice. The rocks of the Greenwich slice exhibit a chaotic style of deformation characterized by pinch-and-swell and pull-apart structures. These rocks lack the well-developed slaty cleavage typical of lithologically similar rocks in the surrounding parautochthonous Lehigh Valley sequence. First-phase deformation is typified by eastnortheast-eastsoutheast-trending folds. A slaty cleavage is axially planar to these folds in the Lehigh Valley sequence and in the incompetent rocks of the Richmond slice, but slaty cleavage is almost totally absent from rocks of the Greenwich slice. Second-phase deformation is characterized by northeast-trending strain-slip and fracture cleavage and minor folds and is well displayed in all rocks of the allochthon. Finally, thirdphase deformation is not well developed, but the structures that appear to be related to this deformation trend eastnortheast. This deformation can be distinguished from the first phase only where cross-cutting relations exist. A conceptual plate tectonic model for the lower Paleozoic section of the central Appalachians has been developed. The model synthesizes structural, sedimentologic, and stratigraphic data of the present study with interpretations and conclusions of other geologists who have worked in the central Appalachian Great Valley. The proposed model explains the distribution of the allochthonous and parautochthonous rocks in the Great Valley in terms of the present-day tectonics of the Banda Sea north of Australia. Late Proterozoic rifting resulted in the formation of a basin separating the North American craton and a microcontinent to the southeast. The microcontinent may be represented by such basement complexes as the Fordham Gneiss and Baltimore Gneiss terranes. From Early Cambrian to Early Ordovician time, a stable carbonate platform was established on the North American craton as evidenced by the Frederick Valley sequence of Maryland and the parautochthonous carbonate rocks of the Pennsylvania Great Valley. Formation of a carbonate platform on the microcontinent is illustrated by the Setters Formation and Cockeyville Marble overlying basement rocks of the Baltimore Gneiss (Thomas, 1977). The Virginville Formation, therefore, may represent Upper Cambrian to Lower Ordovician slope and lower slope rocks that were deposited basinward of a carbonate shelf on the microcontinent. Deposition of the deepwater olistoliths of chert, limestone, and variegated shale and mudstone of the Windsor Township Formation occurred concomitant with E arly Ordovician shelf deposition of the Beekmantown Group.
37
CONCLUSIONS
Subduction of the North American Continent probably began in early Early Ordovician time. Chazyan foundering of the shelf in response to subduction resulted in transgression characterized by deposition of high-calcium limestone such as the Annville Limestone. The trench sediments of the Windsor Township Formation were deposited in the middle-fan area of a large submarine fan. A southeasterly source for these sediments suggests that the fan was adjacent to the microcontinent. Trench sedimentation and melange deformation evidenced by the rocks of the Greenwich slice occurred during Chazyan time and was coeval with foundering of the North American shelf. As subduction continued, the chert, limestone, and variegated shale and mudstone units were detached from the subducting plate by the process of selective subduction and incorporated into the progressively deforming trench sediments. As the North American Continent approached the trench, the continental margin was uplifted as a result of the aborted subduction of the buoyant continental material, and the plates became locked. Erosion of the shelf sediments in response to uplift produced the Black River hiatus. Deposition of the Jacksonburg Limestone to the northwest of the tectonic welt indicates that uplift and erosion were in progress by Rocklandian time. Continued uplift resulted in the transition from the shallow-water "cement limestone" facies to the deeper water "cement rock" facies. The inability of the North American craton to be subducted beneath the microcontinent resulted in late Blackriveran to Kirkfieldian underthrusting and detachment of slabs of cratonic basement on easterly inclined faults. These slabs of basement and associated miogeoclinal rocks are represented by the carbonate nappes in the Lehigh and Lebanon Valleys. Continued convergence from the southeast, possibly as a result of a shift from southeast-dipping to northwest-dipping subduction southeast of the microcontinent, resulted in the thrust emplacement of the Richmond slice onto the Greenwich slice by Edenian time. Sediments of the Martinsburg Formation were shed from the northwestward-advancing wedge of basement, miogeoclinal, and eugeoclinal rocks in a foredeep. As the wedge continued to move, the allochthons became detached from the underlying carbonate nappes and slid into the Martinsburg basin in advance of the nappes. This may have occurred in late Shermanian time although direct evidence is lacking. Nappe formation continued into Maysvillian time by thrusting along faults separating each of the basementcored carbonate nappes. As motion between the two plates ceased, heating and uplift occurred in Maysvillian and Richmondian
time and resulted in the post-Martinsburg deposition (Hudson Valley) unconformity. Later Alleghanian thrust faulting further telescoped the nappes (Drake, 1978) and translated them over the rocks of the Hamburg klippe. REFERENCES CITED Alterman, I. B., 1972, Structure and history of the Taconic and surrounding autochthon, east-central Pennsylvania: Columbia University, unpub. Ph.D. dissertation, 287 p. Bergstrom, S. M., Epstein, A. G., and Epsteiri, J. B., 1972, Early Ordovician North Atlantic Province conodonts in eastern Pennsylvania: U.S. Geological Survey Professional Paper 800-D, p. D37-D44. Berkland, J. 0., Raymond, L. A., Kramer, J. C., Moores, E. M., and O'Day, Michael, 1972, What is Franciscan?: Am. Assoc. Petroleum Geologists, 56, p. 2295-2302. Berry, W. B. N., 1960, Graptolite faunas of the Marathon region, Texas: Univ. Texas Pub. 6005, 179 p. ___1968, Ordovician paleogeography of New England and adjacent areas based on graptolites, in Zen, E-An, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian Geology: Northern Maritime: New York, Wiley Interscience, p. 23-35. Beutner, E.G., 1975, Tectonite and melange A distinction: Geology, 3, p. 358. Bird, J. M., 1969, Middle Ordovician gravity sliding Taconic region, in Kay, Marshall, ed.. North Atlantic geology and continental draft: Am. Assoc. Petroleum Geologists Mem. 12, p. 670-686. Bird, J. M., and Dewey, J. F., 1970, Lithosphere plate-continental margin tectonics and the evolution of the Appalachian orogen: Geol. Soc. America Bull., 81, p. 1031-1060. Bouma, A. H., 1962, Sedimentology of some flysch deposits, a graphic approach to facies interpretations: Amsterdam, Elsevier Publishing Co., 168 p. ___1972a, Recent and ancient turbidites and contourites: Gulf Coast Assoc. Geol. Soc. Trans., 22, p. 205-221. ___1972b, Fossil contourites in lower Neisenflysch, Switzerland: Jour. Sed. Petrology, 42, p. 917-921. Bouma, A. H., and Hollister, C. D., 1973, Deep ocean basin sedimentation: Soc. Econ. Paleontologists and Mineralogists. Pacific Sec. Short Course, Anaheim, Calif., 157 p. Chappie, W. M., 1973, Taconic orogeny: abortive subduction of the North American continental plate: Geol. Soc. America Abs. with Programs, 5, p. 573. Cook, H. E., 1979, Ancient continental slope sequences and their value in understanding modern slope development, in Doyle, L. J., and Pilkey, 0. H., eds., Geology of continental slopes: Soc. Econ. Paleontologists and Mineralogists Spec. Pub. 27, p. 287-305. Cook, H. E., McDaniel, P. N., Mountjoy. E. W., and Pray, L. C., 1972, Allochthonous carbonate debris flows at Devonian ("reef") margins, Alberta, Canada: Bull., Canadian Petroleum Geol., 20, p. 439-497. Cook, H. E., and Taylor, M. E., 1977, Comparison of continental slope and shelf environments in the upper Cambrian and lowest Ordovician of Nevada, in Cook, H. E., and Enos, Paul, eds., Deep-water carbonate environments: Soc. of Econ. Paleontologists and Mineralogists Spec. Pub. 25, p. 51-81. Corbett, K. D., 1973, Open-cast slump sheets and their relationship to sandstone beds in an Upper Cambrian flysch sequence, Tas-
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